1. Field
The present invention relates to an organic light emitting display.
2. Related Art
Recently, as multimedia applications and their use increase, the more important the flat panel displays (FPD) become. Hence, various flat panel displays such as a liquid crystal display (LCD), a plasma display panel (PDP) or an organic light emitting display are used more and more. The organic light emitting display has rapid response time, low power consumption, and self-emission structure.
Furthermore, the organic light emitting display has a wide viewing angle, so that it can excellently display a moving picture regardless of the size of the screen or the position of a viewer. Because the organic light emitting display may be manufactured in low temperature environment and by using a semiconductor fabrication process, the organic light emitting display has a simple manufacturing process. Hence, the organic light emitting display is attractive as a next generation display.
Generally, the organic light emitting display emits light by electrically exciting an organic compound. To display a predetermined image, the organic light emitting display has N×M organic light emitting diodes arranged in a matrix format and may be voltage driven or current driven. The driving methods of the organic light emitting display include a passive type and an active type using a thin film transistor.
A pixel circuit of an organic light emitting display in an active type comprises at least a switching transistor, a capacitor, a driving transistor and a light emitting diode. And, the transistors may comprise a semiconductor layer, which is formed of poly-silicon having excellent mobility.
The semiconductor layer comprising the poly-silicon has non-uniformity in grain size so that the difference of threshold voltage occurs among the driving transistors of each pixel circuit. Therefore, the pixel circuit comprises more transistors in order to compensate for it so that the structure of the pixel circuit has been variously changed.
As described above, when a number of transistors are used in order to compensate for the difference of the threshold voltage among each pixel circuit, aperture ratio is significantly reduced. Also, the storage electrodes of a capacitor are mainly formed simultaneously with a semiconductor layer, a gate electrode or a source electrode and a drain electrode of the transistors. However, as the area occupied by the transistors becomes large, the area on which the capacitor is formed reduces, causing a problem that the performance of the capacitor is deteriorated.